This invention relates generally to the selection and production of color, and more particularly to a method and apparatus for selecting a color according to color characteristics of one color classification system and reproducing the selected color in accordance with color characteristics of a different color classification system.
When a beam of white light is passed through a prism, the beam is spread into a well known band of colors. The colors in the band are fully saturated colors, which are considered "pure" colors, and are generally referred to as the spectral hues. While the band contains a large number of hues that form a continuous spectrum, an observer perceives seven distinct hues identified as red, orange, yellow, green, blue, indigo and violet, which appear to blend smoothly from one into another. The complete range of visible colors contains a vast number of colors in addition to the fully saturated colors or spectral hues. These additional colors can be created by combining appropriate hues in selected proportions.
In practice, however, colors are created in a medium defined by a set of primary colors, with a particular color created as a mixture of the primary colors included in the set. Primary colors are selected according to their ability to create a wide range of colors in a given mixing medium. For example, in projected light media such as the common video cathode ray tube (CRT) display, the additive set of primaries red, green and blue (RGB) are often used, while in reflective media such as the common 4-color printing process, the subtractive set of primaries cyan, magenta, yellow and black are often used. As there are many mixing media, there are many sets of mixing primaries. However, since primary sets mix colors, rather than pure spectral hues, to create other colors, no primary sets can create the complete range of visible colors. The term "color gamut" is used to describe the set of colors which can be created by a particular set of primary colors.
In describing the 4-color printing process, black was identified as a primary of the subtractive set of primary colors. Technically, black is not a color, but is a result of the absence of color. The mixing media of the 4-color printing process utilizes black to create black and shades of colors otherwise not obtainable by mixing the actual colors included in the primaries of the subtractive set. On the other hand, black exists in an additive mixing medium when there is absence of all primaries.
Similarly, white is not a color. In an additive mixing medium, white can be created by combining equal proportions of a pair (or pairs) of complementary colors included in the color gamut obtainable from the primary set. In subtractive mixing media, white exits when there is an absence of all primaries.
While colors may be produced using alternative techniques in various media, broadly speaking, color perception is a subjective visual experience resulting from the stimulation of the retina by light. Visual sensations produced from observation of a given light vary from observer to observer, because the response characteristics of retinas vary from observer to observer. Moreover, the retina is able to distinguish only a small fraction of the colors included in the range of visible colors, and its sensitivity to color is affected markedly by the level of received light. To aid in identifying and distinguishing colors, several color classification systems have been developed to represent colors through models. Each classification system consists of a set of color characteristics, often attributes of appearance of light, used to specify different colors according to varying values of the characteristics.
In some classification systems, colors are classified according to a set of primary colors, each primary of the set being a color characteristic. In these systems, the proportions of primaries determine the color. The aforedescribed CRT display and 4-color printing process are examples of color systems that produce a range of colors that can be described by models based upon such classification systems. However, such color classification systems lack subjective color characteristics colloquially used to describe color and thus are difficult to use. In other classification systems, colors are classified according to characteristics of hue, saturation and value (HSV), the numeric quantities of each determining the color. These classification systems are based upon the intuitive notions of hue, saturation and value. Consequently, models of such classification systems provide a framework for classifying colors in terms of subjective color characteristics that can be dealt with more naturally. While the hue, saturation and value (HSV) color characteristics systems will be discussed at length in connection with the present invention, the characteristic properly designated as "value" will be referred to as "brightness" to avoid confusion with references to numeric value.
Models of such color classification systems are described in Joblove, G. H. and D. Greenbert, "Color Spaces for Computer Graphics", SIGGRAPH 78 Proceedings, published as Computer Graphics, 12(3), August 1978, pp. 20-27 and Smith, A. R., "Color Gamut Transform Pairs", SIGGRAPH '78 proceedings, published as Computer Graphics, 12(3), August 1978, pp. 12-19, which are incorporated by reference herein.
With respect to the various color classification systems, there generally exist mathematical transforms which provide a mapping function between different color classification systems. Such transforms are frequently useful, for they provide a means whereby corresponding color points in different color classification systems may be located. For example, a particular color specified in terms of the color characteristics of a hue, saturation and brightness color classification system can be converted to the corresponding color characteristics of a red, green and blue primary color classification system through use of the associated transform. Such a technique would naturally find a broad range of applications in dealing with color in various media. By way of example with respect to the color video CRT medium wherein color is specified in terms of relative portions of the primary colors of red, green and blue, a particular color specified in terms of the color characteristics of hue, saturation and brightness can be directly converted and displayed in the CRT medium through use of the associated transform to determine the respective amounts of red, green and blue necessary to reproduce the particular color. In a similar manner, for any color displayed in terms of the color characteristics of red, green and blue the corresponding hue, saturation and brightness color characteristics could be likewise obtained to reproduce the color. In dealing with other media for the production of color, such as printing, wherein the color characteristics may be the primaries cyan, magenta, yellow and black, the corresponding amounts for each of the primaries could be determined in a similar manner to reproduce a color specified in terms of hue, saturation and brightness color characteristics.
Numerous techniques have been taken in the past with respect to the process of color selection and production. In one such technique, a user is provided the ability to choose a particular color from a set of pre-defined colors. While the set of predefined colors may be changed, the colors contained within the set are fixed during a selection process to individual and distinct colors. While the number of colors present may be large, the choice of the user is nevertheless restricted during a selection to colors which are present within the pre-defined set of colors. The restriction of freedom of color choice to a group of pre-defined colors is highly undesirable.
In an alternate technique of color selection and production used in the past, color selection is achieved through the display of a set of pre-defined colors along with independent displays of numeric values of hue, saturation and brightness color characteristics associated with a particular color selected from the set. In accordance with this technique, a set of individual, distinct, pre-defined colors are displayed to the user along with a display of three linear lines. Each line represents numeric values associated with one color characteristic. That is, one line represents numeric values of only the hue color characteristic, a second represents numeric values of only the saturation color characteristic and the third represents numeric values of only the brightness color characteristic. Selection of a color starts with the selection of one of the displayed pre-defined colors. This selection causes indications to appear in the three linear lines each independently specifying the numeric values of one of the hue, saturation and brightness color characteristics defining the selected pre-defined color. Colors other than the pre-defined color can be obtained by selecting different numeric values from one or more of the independently displayed numeric values of color characteristics. While the ability to select different numeric values of color characteristics does enable selection of colors in addition to the displayed pre-defined colors, the independent displays of the numeric values of the color characteristics does not permit the user to observe interactions of any combinations of the color characteristics other than the final selected color resulting from the combination of all color characteristics, namely, hue, saturation and brightness. Moreover, this technique does not offer to the user the ability to view all colors which are present in the color gamut. Only the set of pre-defined colors and the selected color (if different) are displayed. The limited display of colors also fails to display any of the colors adjacent to the selected color, because the adjacent colors invariably are located in the color gamut between the selected color and pre-defined colors.